This invention relates to electric power plants of the type employing fossil fuel fired steam generators and, more particularly, to a control system for such a power plant. In power plants employing fossil fuel fired steam generators, variable pressure operation at the turbine throttle, particularly for cycling service, is desirable to minimize steam temperature effects which could otherwise lead to turbine damage. In the past, steam generators for cycling service have been of the natural circulation type. However, the development of the integral steam separator has now made the once through supercritical steam generator also suitable for cycling service and variable pressure operation. The once through steam generator is normally arranged with a start-up bypass system designed to recirculate at low loads. It has been the practice on these units to operate at variable pressure and to come to full throttle pressure at 25 percent load at which point the bypass system was phased out of service. Recently, it has been proposed, in order to achieve maximum protection against turbine damage, to operate at variable throttle pressure up to a higher load by having some of the turbine throttle valves fully opened, for example, the first two for a four valve machine, so that 60 percent load is achieved at full throttle pressure. Alternatively, variable pressure can be carried to full load.
The control system of the present invention is designed to control the power plant employing a once-through steam generator in order to achieve variable pressure operation at the turbine throttle up to any selected load.